1. Field of the Invention
This invention relates generally to an assembly for transferring loads between adjacent cast-in-place slabs, and more particularly, to an improved system for transferring a load across a joint between a first and a second slab, the load being applied to either slab.
2. Related Art
Typical floors in industrial buildings, roads, driveways, sidewalks, and other, are constructed using concrete. However, in the curing process, concrete shrinks and internal stresses develop, negatively affecting the performance of such floors. To overcome the concrete shrinkage problem, joints or breaks are inserted in the concrete, as shown in FIG. 1A. The concrete floor is divided into a series of blocks or slabs. Joints, typically spaced at 20 ft, accommodate any concrete shrinkage at the expense of breaking the continuity of the floor. To restore the continuity of the slab at a joint, load transfer devices can be used to distribute the applied loads between the first and second slabs across the joint.
Several steel dowel bars or plates were proposed to bridge the joint gap between adjacent concrete slabs. Traditional round steel dowel bars, as shown in FIG. 1B have the main disadvantage of restraining relative movement of the adjacent slabs parallel to the joint. As round steel dowels are all around encased in concrete, any relative movement between concrete slabs parallel to the joint is prevented, as shown in FIG. 2B. This contributes to the formation of restraint cracks in the concrete. In addition, any misalignment in placement of round dowels can lock the joint by preventing any movement perpendicular to the joint surface from occurring, as shown in FIG. 2A.
U.S. Pat. No. 4,733,513 and No. 6,145,262 issued to Schrader et al. introduced square steel dowel bars, as shown in FIG. 1C, equipped with sheaths and compressible material along the vertical sides. The use of a combination of square dowels and compressible material allowed for some relative movement between adjacent slabs parallel to the joint, but to a limited extent. The compressible material within the sheaths did not provide enough tolerance for relative movement parallel to the joint.
Given that most load transfer occurs in the vicinity of the joint, a major shortcoming of previously disclosed dowels is the use of a dowel with homogeneous section. Previously disclosed dowels placed relatively insufficient steel material along the joint where most of the load transfer occurs, and more than required material away from the joint, where the dowel is relatively minutely loaded.
U.S. Pat. No. 6,354,760, No. 7,481,031, No. 7,716,890, and No. 0,014,018 issued to Boxall et al., disclosed the use of diamond, tapered, and rectangular plates, respectively, for load transfer, as shown in FIGS. 3A-3C. The introduction of steel plates for load transfer enabled an increase in the contact area between the steel dowel and concrete, thus reducing the corresponding stresses along the aforementioned contact area. Furthermore, the design of sleeves for plate dowels provided some extra space along the vertical sides of the plate in order to allow for relative slab movement parallel to the joint. As a result, steel plates were able to slide parallel to the joint within the corresponding sleeve without any restraining force.
Diamond dowels, that constitute square steel plates with their largest dimension, or diagonal, positioned along the joint line, as shown in FIG. 3B, are common in the industry. In addition to the above mentioned advantages, the shape of diamond dowels is optimized in order to provide the highest amount of steel material in the highly stressed region in the vicinity of the joint and less steel material away from the joint where stresses are reduced. However, the performance of diamond dowels is limited by several factors, particularly for relatively wide joint widths. Due to the tapered vertical sides of the diamond dowel, and the fact that the dowel is embedded in the concrete on at least one side, the concrete-steel contact area drops as the joint width increases. Hence, diamond dowels become less effective in transferring the load across the joint between adjacent slabs as the joint gap widens.